The investigation concerning microcapsules, wherein a core material is contained inside the capsule, has been conducted in various fields such as in recording/display materials, drug delivery systems (DDS), cosmetics/perfume, food products, and agricultural chemicals. In one such example, an attempt was made to improve the drug stability in the product by enclosing the drug inside a capsule.
As such a microcapsule, a silica microcapsule has been known. There have been numerous reports concerning the production methods of silica microcapsules, for example, phase separation method, drying-in-liquid method, spray drying method, interfacial polymerization method, and in-situ polymerization method. However, in order to obtain silica microcapsules containing a core material, a separate treatment such as the impregnation, in a solution wherein the core material has been beforehand dissolved, of hollow silica microcapsules is usually necessary.
On the other hand, a one-step preparation method of the silica composite microcapsules containing a core material has been reported (for example, refer to patent literature 1). The preparation is done by the polymerization of silica, at the water-oil interface, using a water-in-oil type (W/O type) emulsion wherein the aqueous phase containing the core material is dispersed. In this method, however, a water-insoluble tetraalkoxysilane is used; thus the reaction system needs to be a water-in-oil type (W/O type). Therefore, there was a disadvantage in that the form of the obtained formulation is restricted.
In cosmetics and pharmaceuticals, various thickening agents and gelling agents have been used in order to maintain product forms. In the past, as water-based thickening/gelling agents, for example, natural water-soluble polymers such as agar and gelatin and synthetic water-soluble polymers such as polyethylene glycol and acrylic acid polymers were suitably selected and used depending upon respective purposes and effectiveness. However, when the base material was solidified with the use of these water-soluble polymers, there were usability problems in that the skin compatibility was poor or the spreadability during application was poor. In addition, when used as cleansers for cleansing, satisfactory cleansing power could not be obtained.
On the other hand, alkoxysilanes such as tetraethoxysilane are known to form silanol groups by the hydrolysis of alkoxy groups and to form silica gel by their subsequent dehydration-condensation. However, most of conventionally used alkoxysilanes are insoluble in water, and the hydrolysis reaction does not automatically proceed when an alkoxysilane is added into water. Thus, it is necessary to add other additives, and alkoxysilanes are not suitable for the gelation of an aqueous base material. In recent years, a simply mixed aqueous solution of a water-soluble polyhydric alcohol-substituted silane derivative was found to form a solid monolithic silica gel. For example, the application to a silica gel precursor for chromatography or the application to a biosensor, on which a biological component such as an enzyme is immobilized, is reported (for example, refer to patent literature 2 and non-patent literatures 1 to 4). However, the use of such a polyhydric alcohol-substituted silane derivative has not been tried as an aqueous gelling agent.    [Patent Literature 1] Japanese unexamined patent publication No. 2001-38193    [Patent Literature 2] PCT international publication No. WO03/102001    [Non-patent Literature 1] Sattler et al., Ber. Bunsenges. Phys. Chem., 1998, 102, 1544-1547.    [Non-patent Literature 2] Mayer et al., J. Phys. Chem. B, 2002, 106, 1528-1533.    [Non-patent Literature 3] Schipunov, J. Colloid and Interface Sci., 2003, 268, 68-76.    [Non-patent Literature 4] Schipunov et al., J. Biochem. Biophys. Methods, 2004, 58, 25-38.